Synthesis of Erythrina and related alkaloids. XXIII. Intramolecular cyclization approach. (2). Synthesis and reactions of 1,7-cyclo-cis-erythrinans, potential intermediates to natural Erythrina alkaloids.

Abstract

Several 2, 8-dioxo-1, 7-cycloerythrinan drivatives (bearing a 6β-ethoxycarbonyl or 6β-hydrogen substituent) were prepared in good yields from the reported 2, 8-dioxo-7β or 7α-hydroxyerythrinan derivatives by a base-catalyzed intramolecular alkylation of the corresponding O-mesylates, and they were shown to be useful intermediates for synthesizing natural erythrinan alkaloids. The C-1 of these compounds has been suitably protected for further manipulation at C-3 and the cyclopropane ring can be readily cleaved in a reductive or a non-reductive manner to give C-1 methylene or C-1 olefin derivatives. The latter process is discussed in detail with reference to several examples of ionic and radical opening. The reactivity of the carboxylate ester group on the cyclopropane ring bearing an electro-negative substituent was unusually high.